A Real-Time phenomenon involves two operations ordered in real time, as measured by imaginary, perfectly synchronized wall clocks. One event must complete before the other begins: a real-time dependency.
Real-Time phenomena always involve some other relationship between the operations. For instance, a Stale Read occurs when a read begins after a write completes, but fails to observe the write’s effects (e.g. the write rw-depends on the read)). Jepsen calls this G2-item-realtime, by analogy with G2-item. You could also imagine a system which ensures that if one write begins after another ends, those writes will logically take effect in that order. Such a system would prevent G0-realtime: a cycle composed of write-write and real-time edges.
More formally, we say that given some phenomenon P defined in terms of a cycle in a dependency graph G, the phenomenon P-realtime is the corresponding cycle in the graph (G ∪ R), where R is the graph of real-time dependencies between transactions in G.
Linearizable systems proscribe Real-Time phenomena. The “Strong” variants of transactional consistency models generally differ from their non-strong counterparts by ruling out Real-Time phenomena as well. For instance, Serializable proscribes G1c and G2: cycles involving write-write, write-read, and read-write dependencies. Strong Serializable goes further by proscribing G1c-realtime and G2-realtime: cycles involving write-write, write-read, read-write, and real-time dependencies. Similarly, Strong Snapshot Isolation strengthens Snapshot Isolation by ruling out G-nonadjacent-realtime.
Literature
This concept is adapted directly from Adya’s Weak Consistency, section 4.9, which defines Strict Serializability in terms of an extension to the Direct Serialization Graph used to define Serializability:
Consider a graph called the Real-time Serialization Graph or RSG which is the same as the DSG except that there are some extra edges to capture the order of non-overlapping transactions: If a transaction Ti commits before Tj executes its first event in real-time, we add a real-order edge from Ti to Tj; these extra edges force non-overlapping transactions to be serialized in the order in which they executed in real-time. Strict serializability or PL-SS is defined as the level that disallows G1 and G2 when these phenomena are defined on the RSG instead of the DSG.
See Also
A Stale Read is a Real-Time phenomenon where a later read fails to observe the effects of an earlier write.